The invention relates to a method for transmitting a row of data elements, which row comprises normal data elements and control data elements, the method comprising:
detecting said control data elements within said row, and
marking each control data element by supplementing a supplementary data element tot the row to be transmitted.
Such a method is known from practice.
When transmitting rows of data elements, the problem arises that to specific data elements there may be assigned, by some equipment, a specific control significance, or that for technical reasons a specific data element may not be included. Thus, the hexadecimal code xe2x80x9cFFxe2x80x9d (corresponding to the binary code xe2x80x9c11111111xe2x80x9d, and therefore a byte having only ones) may be interpreted by equipment as a control character. If such a control data element is present among the data elements to be transmitted, it may cause problems in the transmission, since the data elements are not transparent for the equipment, i.e., may affect the operation of the equipment.
It is known to mark such control data elements at the transmitting end of the transmission route, e.g., by doubling them. In the above example, the data element xe2x80x9cFFxe2x80x9d would be doubled to xe2x80x9cFFFFxe2x80x9d. In this connection, the equipment in the transmission route is constructed such that all double control data elements are not interpreted as a control character and the like. At the receiving end of the transmitting route, the double control data elements are unpacked to render single control data elements: xe2x80x9cFFFFxe2x80x9d is returned to xe2x80x9cFFxe2x80x9d.
Said known method has the drawback that the number of data elements to be transmitted may increase essentially, since each control data element is doubled. In addition, it is necessary that all equipment present in the transmitting route is arranged for ignoring double control data elements and solely interpreting several control data elements. Since the control data elements in this known method are transmitted anyway, errors may easily occur here.
An object of the invention is to avoid the drawbacks of the prior art and to provide a method for transmitting data elements which enables excluding control data elements from the transmission in a reproduceable manner.
A further object of the invention is to provide a method for transmitting data elements which enables excluding control data elements from the transmission without essentially increasing the number of data elements to be transmitted.
For this purpose, a method of the type referred to in the preamble is, according to the present invention, characterised in that
the supplementary data element comprises a positional indication to the first control data element in the sequence of data elements of the row,
each control data element is replaced by a replacement data element which comprises a positional indication to next control data element, and
a replacement data element does not correspond to a control data element.
By replacing each control data element by a replacement data element, which in itself is no control data element, all control data elements are removed from the row. By additionally, at the location of the removed control data elements, applying replacement data elements which contain a positional indication, it is possible at the receiving end to determine where control data elements have been removed from the row. Since basically there is required only one single supplementary data element for transmitting a positional indication of the first control data element occurring in the row, the row increases by one data element only, irrespective of the number of control data elements.
It is noted that from U.S. Pat. No. 5,363,098 discloses some analogies with the present invention, disclosing a method for compressing, decompressing and manipulating bit-maps. The bit-map is aligned along byte boundaries. Each aligned byte is classified as a gap byte (GBYTE) if all of the bits of the byte store the same logical value. Otherwise the byte is classified as a map byte (MBYTE). Adjacent bytes of the same class are grouped. Groups of gap bytes are encoded into an atomic sequence of bytes as a count of the number of bits included in the grouped gap bytes. Map bytes are duplicated in the atomic sequence and also associated with a count thereof. Said disclosure does not, as the present invention does, address the problem of (control) characters like the hex code xe2x80x9cFFxe2x80x9d, which may control or affect the operation of computer equipment.
The method according to the invention is preferably carried out in such a manner that the positional indication to next control data element comprises a relative position with respect to a replacement data element. By applying a relative position, i.e., with respect to the preceding replacement data element, instead of an absolute position, i.e., with respect to the supplementary data element, the positional indication will generally have a smaller value, so that fewer bits may suffice.
If the positional indication to next control data element is not capable of being accommodated in a supplementary or replacement data element, the method according to the invention is advantageously carried out in such a manner that at least a further supplementary data element, which contains a further positional indication, is inserted into the row. The further positional indication may indicate the position of the next control data element, or the position of another further supplementary data element. In this manner, there may also be indicated the positions of control data elements far removed from one another, whose positional indications might exceed the number of bit positions available in a supplementary data element.
In this connection, the method is advantageously carried out in such a manner, that the at least one further supplementary data element in the row is inserted at the farthest position whose positional indication is capable of being accommodated in a supplementary or replacement data element. As a result, the position of a control data element is capable of being indicated by as few further supplementary data elements as possible.
It is also possible to carry out the method according to the invention in such a manner, that a supplementary or replacement data element, apart from a positional indication, also comprises a length indication which indicates the number of data elements taken by the positional indication. Using such a length indication, the number of bit positions available to the positional indication might be expanded in a simple manner.
The control data elements may all be of the same type. That is to say, in fact there is only one control data element, however, which may occur several times in a row.
It is also possible that the control data elements are of several types. In that case there are several, mutually different, control data elements which each may occur once or several times in a row. The method according to the invention is then advantageously carried out in such a manner that a replacement data element additionally comprises a type indication of a replacement control data element. The type indication is advantageously chosen in such a manner that correspondence with a control data element is excluded. In other words, the type indication has a (bit) pattern which, irrespective of the value of the positional indication, is not capable of providing any control data element.
In order to prevent that a positional indication, in combination with non-control data elements, might unintentionally form a control data element, the method according to the invention is advantageously carried out in such a manner, that a data element is marked as a control data element if the data element corresponds to part of a control data element. In this connection, the length of this part plus the length of the same data element preferably equals the length of a control data element.
For the sake of security, it is preferably assumed in advance that a positional indication is part of a control data element.
In order to prevent that at least part of the supplementary data element is capable of being replaced by a receiving data element, as a result of which the positional indication in question might be overwritten, the method according to the invention is preferably carried out in such a manner that, if the positional indication present in the supplementary data element has a smaller value than the length of the supplementary data element, the value is increased by an auxiliary value to said length, and that the replacement data element in question replaces the data element which begins at the position adjacent to the supplementary data element. In doing so, the length of the replacement data element is preferably reduced by the auxiliary value.
A data element may basically comprise a random, preferably predetermined, number of bits. The invention may also be applied to situations in which a data element comprises exactly eight bits (one byte).
A further object of the invention is to indicate devices and a system for implementing the method.